Digital images based on x-rays, magnetic resonance, optics and other modalities are widely used in the medical field. For example, x-ray images are used by radiologists to screen for calcifications, lesions, masses or patterns which may be indicative of cancer. It can therefore be appreciated that the accuracy of a screening is dependent upon the image.
It is known that noise can produce undesirable artifacts in images. For example, correlated noise introduced during x-ray image capture and readout can cause distinct bands across an image. Electronic x-ray detectors can be divided into two classes—those in which direct methods are used to convert x rays into an electric charge and those in which indirect methods are used. Both direct- and indirect-conversion detectors accumulate and store the electronic charges received during x-ray exposure. In direct-conversion detectors, an x-ray photoconductor such as amorphous selenium directly converts x-ray photons into an electric charge. In indirect-conversion detectors, x-ray energy is converted into visible light using scintillators and then the light energy is converted into an electronic charge using photodetectors such as amorphous silicon photodiode arrays or CCDs. Typical x-ray detectors of either type (direct or indirect) are divided into an array of M rows by N columns (also referred to as channels) of picture elements (i.e., detector pixels). For example, a mammography detector may include 4096 rows and 3584 channels per row. An example detector array 10, comprised of rows 12 and channels 14, is shown in FIG. 1. Each channel is coupled to one of the charge amplifiers 16, while each row is coupled to one of the row drivers 15. During x-ray exposure, the electric charge at each pixel location of the detector array is accumulated and stored. The electric charge relates to the number of photons received at the pixel during the x-ray exposure and is representative of the attenuation experienced by the individual transmitted x-rays as they traveled through an imaged object. Following x-ray exposure the electric charge pattern of the detector is read out and converted to a digital image, where the brightness of the individual pixels of the digital image directly relates to the electronic charge of the detector pixel and enables a visual representation of the internal structure of the imaged object. During detector readout, rows are sequentially accessed by the row drivers, and the charge of a pixel in each channel of the accessed row is sampled by charge amplifiers 16. However, external noise can influence the pixel charge, and the noise effect is correlated when sampling times are correlated. This is problematic because correlated noise phase differences between rows appear as bands across the resulting image. It is thus desirable to reduce the effects of correlated noise in captured images for various modalities.